Skeletal muscle regeneration occurs during normal exercise and aging, and in disease states such as muscular dystrophies. Satellite cells, which reside adjacent to the myofiber under the basal lamina, are required for normal muscle regeneration. Following injury, quiescent satellite cells undergo proliferation and differentiation into myoblasts that fuse to form new myofibers. Recent evidence suggests that satellite cells are heterogeneous and may not undergo self-renewal but may be renewed from a muscle stem cell. We have identified a subpopulation (satellite-SP) within the muscle side population cells that express the satellite cell markers Syndecan-4 and Syndecan-3, as well as the stem cell markers ABCG2 and Sca-1. Satellite-SP cells possess myogenic potential;they are capable of forming multinucleated myotubes in culture and home to the satellite cell niche following transplant with concurrent injury into recipient mice. Additionally, the satellite-SP cell undergoes asymmetric cell division where one daughter cell becomes an activated satellite cell and the other daughter cell undergoes self-renewal. We hypothesize that satellite-SP cells function as a satellite cell progenitor and replenish the satellite cell compartment. The major objective of this proposal is to test the myogenic potential of the satellite-SP cell and its role as a muscle stem cell. Specifically, 1) the expression of ABCG2 relative to satellite cell activity using an ABCG2-GFP mouse will be characterized. 2) The requirement for ABCG2 function in muscle stem cells will be tested by analyzing the Abcg2 null mouse during muscle regeneration. 3) The progeny of ABCG2+, Syndecan-4+ satellite-SP cells will be followed using lineage tracing to determine their ability to replenish the satellite cell pool and to contribute to newly regenerated myofibers. This work will further contribute to the understanding of the adult origin and maintenance of satellite cells and has significant relevance to the development of stem cell based therapies for the treatment of muscular dystrophies. The goal of this proposal is to understand the cellular characteristics and myogenic potential of newly identified muscle progenitor cells. These cells may represent a potential source for use in cell-based therapies to treat muscular dystrophies or other degenerative muscle diseases.